New discovery in high-temperature superconductors: Sub-unit-cell scale pair density modulation

Superconductivity in quantum materials, irrespective of whether the Cooper pairing on the Fermi surface is mediated by phonons or electronic fluctuations, is described based on the Bardeen–Cooper–Schrieffer (BCS) theory of zero momentum Cooper pair condensation on the crystal lattice of the superconductors, following the lattice symmetry. In recent years, an exotic superconducting state with the finite momentum Cooper pairing known as the pair density wave (PDW) state, which breaks the translational symmetry and shows spatially periodic modulations of the superconducting order parameters, has attracted great research interest. The experimental evidence of PDW has been detected in unconventional superconductors, like cuprates and Fe-based high-temperature (high-T_c) superconductors, with the modulation period spanning several unit cells. However, the pair density modulation within single unit cell, which can provide microscopic insights for the unconventional Cooper pairing on the sub-unit-cell scale, has never been carefully investigated so far.

Using scanning tunneling microscopy/spectroscopy (STM/S), the research team led by Prof. Jian Wang (Peking University) conducted precise atomic-scale measurements on high-quality monolayer Fe(Te,Se) and FeSe superconducting films grown on SrTiO₃(001) substrates. These films possess the highest superconducting transition temperature (approximately 60 Kelvin, or -213°C) among iron-based superconductors. By performing experiments with extraordinary high spatial resolution, the researchers captured clear variations in superconducting properties at different atomic sites within a single lattice unit cell.

The study revealed that the size of the superconducting gap and the sharpness of coherence peaks exhibit periodic spatial modulations with the same period of the crystal lattice. Crucially, these modulations strictly correspond to the crystal lattice structure, with maxima and minima precisely located at the crystallographic positions of the chalcogen atoms, revealing the breaking of the glide-mirror symmetry introduced by the SrTiO₃ substrate. This phenomenon indicates that the chalcogen atoms and their p-orbitals play an essential part in the local Cooper pairing and phase coherence establishment.

"This discovery allows us to see the fine structure of the superconducting state at the atomic scale for the first time," said the researcher. "It's like observing the 'dance' of superconducting electron pairs, where the chalcogen atoms act as the conductors." Previously, the role of chalcogen atoms in the Cooper pairing of iron-based superconductors was often underestimated in theoretical studies, but this new finding will prompt scientists to reconsider the microscopic mechanisms of the unconventional superconductivity in iron-based superconductors.

This research expands the experimental investigation of the pairing mechanism to the sub-unit-cell scale, and opens a new pathway for understanding the pairing mechanism of unconventional superconductors whose crystal unit cell contains multiple atoms. The research team plans to extend this approach to other superconducting systems and explore how atomic-scale information can help us understand the mysterious high-T_c superconductivity.